Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, a cardiac defibrillator. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Implantable medical devices, such as cardiac defibrillators and cardiac pacemakers, are implanted within the body of a person so that battery powered electronic components therein are electrically interconnected with the body. Defibrillating or pacing pulses emitted by the components are applied to stimulate selected body tissue. At the same time, the condition of the stimulating activity within the body can be fed back to the electronic components for sensing.
Typically, an implantable medical device includes thin, oval- or rectangular-shaped can, or case, which is surgically implanted beneath the skin in the chest region or the abdomen. The electronic components, together with batteries for powering the same, are mounted on the inside of the can, and are coupled by a feedthrough assembly to a connector assembly on the outside of the case. One or more leads received by the connector assembly electrically couple the electronic components to transvenous connection of the lead within the person.
In implantable medical devices of this type, the electronic components are internally packaged within the case by a support structure which serves not only to mount the components but to provide some resistance to shock and vibration as well. The support structure may be made of resilient material, such as silicon rubber, which electrically insulates the components from the case, as well as providing some shock and vibration resistance. Such resistance, however, is limited in view of the relative hardness of the material and the typical gasket-like configuration thereof which is lacking in dimensional detail so as to provide little conformance to the components. Still other support structures have been made of rigid thermoplastic material, such as liquid crystal polymer, which, among other things, has proven capable of withstanding the temperatures of almost 600.degree. F. present during welding of the case. While such rigid thermoplastic materials may be formed so as to provide pockets and otherwise more closely conform to the shape of the components, they have been found to provide relatively little in the way of resistance to shock and vibration. Additionally, these precisely formed rigid pockets do not conform to the size tolerance variations of the components. One lot of components may be loose in the pockets and rattle, while those from the next lot may be excessively tight.
A major problem with support structures heretofore used in implantable medical devices is the tendency of the material to outgas chemical products after the device is assembled and placed in use. Because the materials of the support structure are typically made in the presence of non-inert chemicals, outgassing of such chemicals after the device is completed and placed in use, interferes with and eventually contaminates the electronic components.
In addition to the use of resilient materials, such as silicon rubber and relatively rigid thermoplastic material in the formation of support structures for implantable medical devices, it is known in the art to encapsulate electronic components by injecting foam-like materials into a cavity in which the electronic components are placed. In the case of integrated circuit boards, for example, it is known to position the board or portions thereof within a cavity of a mold and to introduce foam so as to surround and encapsulate the board. Examples of these techniques are provided by U.S. Pat. No. 4,250,347 to Fierkens, issued Feb. 10, 1981; U.S. Pat. No. 5,254,501 to Tung, issued Oct. 9, 1993; and U.S. Pat. No. 5,018,003 to Yasunaga et al., issued May 21, 1991.
In spite of this prior work, however, the field of implantable medical devices has heretofore been without the benefit of a support structure having the necessary characteristics of being able to package the internal components in a conforming, shock and vibration resistant fashion, and with relative lightness in weight. In particular, chemical inertness has not been a characteristic of prior art support structures.
Consequently, it would be highly desirable to provide a support structure made of compliant material which avoids such undesirable outgassing effects in addition to providing superior shock and vibration resistance. The support structure should also be relatively light in weight, inasmuch as certain applications of the devices, particularly in the case of cardiac defibrillators, include relatively heavy batteries and other components which can add significantly to the weight of the device.